This project is a multidisciplinary study of breast cancer in which epidemiologic, virologic, immunologic, and pathologic techniques are applied to the comparison of breast cancer in the United States and Tunisia. Pathologic studies supported the concept of rapidly progressing breast cancer (RPBC) or pousee evolutive (PEV) as a separate biologic entity with the demonstration that even PEV-1, which has no clinical evidence of inflammation but which is based primarily on the patient's subjective report of rapid tumor growth, has a significantly higher frequency of nuclear grade 3 than the non-PEV (PEV-0) cases. The similarity between the more advanced RPBC cases and the so-called inflammatory breast carcinoma in the United States was demontrated by the frequent finding of lymphatic permeation involving the skin. Hormone receptor studies demonstrated lower levels in PEV cases vs. PEV-0 and American cases, again supporting the designation of PEV-1 as a biologic entity. A correlation between estrogen receptor levels and a favorable response to treatment was observed.